Biomedical Engineering Reference
In-Depth Information
between the friction and wear behavior of enamel and erosion time. The friction
coeffi cient appeared to increasingly fl uctuate with erosion time (Fig.
4.20
). The
mean coeffi cient increased rapidly at the early stage of erosion, but slowed down
after 5 min (Fig.
4.21
). The same trend happened to both the depth and the width of
the scratches (Fig.
4.22
). That is to say, the infl uence of enamel erosion on its tribo-
logical behavior strongly depended on the time of erosion. At the initial stage of
erosion, partial demineralization, the dominant mode of surface lesion, resulted in
minute etch pits on the surface of the enamel (Fig.
4.19
), breaking the integrity of
the enamel structure and then increasing the surface roughness (Fig.
4.16a
). As a
result, the friction coeffi cient increased, accompanied by fl uctuations. On the other
hand, partial demineralization also caused a decrease in the surface hardness
(Fig.
4.18
), decreasing the resistance against wear. Thus, both the depth and the
width of the scratches increased. As the erosion time increased further, the outer
enamel was lost continuously, and the hardness of the remaining enamel stayed
almost constant except for a slow decrease caused by layer-to-layer variations.
Given that the remains of the outer enamel should be mostly soft organic substances
after 5 min of erosion, a possible mechanical function of the honeycomb-like struc-
ture might be related to a form of solid lubrication. Therefore, the friction coeffi -
cient as well as the depth and the width of the scratches tended to increase only
slowly at the late stage.
4.5.2
Effect of Erosion Location on the Tribo-Erosive
Properties of Enamel
In this study, we studied the erosion behavior of human tooth enamel at different
depths in citric acid solution to explore the relationship between the tribo-erosive
properties of enamel and its location along the long axis direction of a tooth [
56
].
Six teeth were used to prepare enamel specimens [
56
]. Before erosion tests,
about half of the enamel surface of each specimen was coated with acid-resistant
nail varnish and was used as a control. Six specimens were individually exposed to
citric acid solution for 3 min and agitated with an automatic stirrer. After 3 min of
erosion, the specimens were taken out from the solution, and then carefully washed
with deionizsed water and nail varnish remover to remove any residual acid and the
nail varnish on their surfaces. As a result, two different areas appeared on the sur-
face of each specimen: One half was the original surface (this surface was referred
to as the “control”), while the other half was the acid-eroded surface (this surface
was referred to as “eroded”), as shown in Fig.
4.23
. The surface morphology, hard-
ness, and profi le of three specimens, which were randomly selected from all six
specimens, were measured on both the control and the eroded surfaces to study the
surface loss of the exposed enamel. In order to investigate the infl uence of enamel
erosion on its subsequent friction and wear behaviors, the other three specimens
were used to do nanoscratch tests, which were conducted on both the control and the
eroded surfaces of each specimen. Erosion tests were fi rst performed on the surface
